Variable band pass audio filter



July 23, 1963 T. w. HOLDEN 3,

VARIABLE BAND PASS AUDIO FILTER Filed Jan. 12, 1962 2 Sheets-Sheet l INVENTOR. 74 0 7114.5 hf/Va /aa2z w Zia/ 6 40M- A TTORNE YS 1963 T. w. HOLDEN 3,098,987

VARIABLE BAND PASS AUDIO FILTER Filed Jan. 12, 1962 2 Sheets-Sheet 2 Wm! m INVENTOR 7%245 11/66/1672 A TTORNE YS United States Patent Ofiice Patented July 23, 1963 VARIABLE BAND PASS AUDIQ FILTER Thomas W. Holden, Sill W. 16th Place, (Ihicago Heights, Ml.

Filed Ian. 12, 1%2, Ser. No. 165,881 7 Claims. (Cl. 333-71) This invention relates to an audio frequency band pass filter adapted for use in radio-telegraph receivers and the like.

It is an object of the present invention to provide a band pass audio filter which will almost completely reject undesired frequency components while allowing the desired frequency components to pass.

It is a further object of this invention to provide such a filter having means for changing the band width thereof.

A further object is to provide a selectively operative means for injecting the input frequency components into the output circuit.

Still another object of the invention is to provide a band pass filter wherein the amplitude of the output voltage may be limited to a predetermined value.

Other objects, features and advantages of the present invention will be apparent from the following detailed description taken in connection with the accompanying drawings, in which:

FIGURE 1 is a somewhat diagrammatic side elevational view of a structure in accordance with the present invention with certain parts broken away and in section;

FIGURE 2 is a vertical sectional view taken generally along the line IIII of FIGURE 1, and with portions of the filter mechanism shown in vertical section for clarity;

FIGURE 3 is a vertical sectional view taken generally along the line III-III in FIGURE 1 and looking in the direction of the arrows;

FIGURE 4 is a somewhat diagrammatic electric circuit diagram for the embodiment of FIGURES 1-3; and

FIGURE 5 illustrates an alternative permanent magnet construction for use in the embodiment of FIGURES 14.

As shown on the drawings:

As seen in FIGURES 1 and 2, a band pass audio filter in accordance with the present invention may comprise a vertical panel or plate of magnetic or non-magnetic material secured to a base plate '11 by means of brackets 12 and 13. The base plate may be of suitable magnetic material such as soft steel.

As best seen in FIGURE 3, a pair of fastening members 15 and 16 of non-magnetic material extend through apertures in the base plate 11 and through a pair of plates of magnetic material indicated at 2.0 and 21 which form parts of the magnetic circuit of the device. A permanent magnet 22 is confined between the base '11 and intermediate plate 20, and a second permanent magnet 23 is illustrated as being confined between the intermediate plate 24 and the upper plate 21. The permanent magnets 22, and 23 may be magnetized in the vertical direction as viewed in FIGURE 3 and may have polarities as indicated by the letters N and S.

As best seen in FIGURES 1 and 2, the intermediate plate has a pair of vibrating reeds 25 and 26 of magnetic material clamped thereto by means of a bar 27 and screw means 28 and 29.

The upper plate 21 has a further plate of magnetic material 32 secured thereto by means of the nuts $3 and 34 threaded onto the ends of the bolts 15 and 16. The plate 32 extends from the upper plate 21 in the same direction as the reeds 25 and 26 and has an adjustment plate 37 of magnetic material secured thereto by means of a pair of screws 38 and 39. The plate 37 is sufficiently flexible so as to be deflected by means of an adjustment screw 40 which is threaded into the mounting plate '32 and has its lower end 490, FIGURE 1, abutting the upper surface of the adjustment plate 67.

Similar adjustment plates indicated at 43 and 44 are shown in FIGURE 2 as lying adjacent the base 11 and being secured thereto by means of screws such as 46 and 4-7 shown in FIGURE 1 and 49 shown in FIGURE 2. Adjustment screws are indicated at 50 and 51 in FIGURE 2 which are threaded into the base 11 and have their upper ends abutting the lower surfaces of the respective adjustment plates 43 and 44- to deflect the same upwardly away from the base 11.

Mounted on the adjustment plates 43 and 44 are a pair of rectangular core members '55 and 56 which are encircled by respective helical coils indicated at 57 and 58.

Each of the cores 55 and 56 has a non-magnetic coating at its upper end as indicated at '60 and 61 which prevents actual contact between the magnetic material of the reeds 25 and 26 and the respective core members 55 and 56.

The adjustment plate 37 carries a magnetic core mem ber 65 of rectangular configuration having a helical winding 66 encircling it. The core 65 has its broad cross sectional dimension extending transversely to the broad cross sectional dimensions of the cores 55 and 56 so that the end face of the core 65' overlies both of the reeds .25 and 26 as best seen in FIGURE 2. The end face has a non magnetic coating 67 for preventing contact of the reeds 25 and 26 with the end face of the core member 65.

For adjusting the band of the filter, a permanent magnet 76 is mounted in direct proximity to the magnetic circuits linking the reeds 25 and 26. The magnet may be mounted on a shaft 71 of magnetic or non-magnetic material and circular cross section carried by means of a support bracket 72 of magnetic or non-magnetic material. The bracket 72 is shown as being secured to the base 11 by means of a pair of screws 73 and 74, and a pair of collar members 76 and 77 are shown as clamped to the shaft 71 by means of screws 78 and 79 at either side of the support bracket 72. By loosening the screws 78 and 79, the shaft 71 may be adjusted axially to position the magnet 70 in a precise desired position. The screws 78 and 79 are then tightened to prevent axial shifting of the magnet 70 while permitting rotation of the shaft 71 in its bearing within support post 72. A knob is indicated at 80 at the exterior side of the panel 10 for rotating the magnet 70 about the axis of shaft 71 to adjust the strength of the magnetic field produced thereby which impinges upon the magnetic circuits of the filter.

FIGURE 5 illustrates a modified permanent magnet 70" which may be secured to the shaft 71 and positioned so that its magnetic field will adjustably influence the magnetic circuits of the filter. The magnet 70' may be \of circular cross section and magnetized along a diametric plane so that rotation thereof will vary the strength and polarity of the magnetic field impinging on the magnetic circuits associated with the reeds 25 and 26. The means indicated at '83 in FIGURE 2 and 83 in FIGURE 5 securing the magnets 70 or 7 0 to the shaft 71 may comprise a suitable glue or low melting point solder.

Referring to FIGURE 1, the cylindrical hole 87 in panel 10 may receive a suitable phone jack such as indicated at 88 in FIGURE 4. The hole 89 in panel 10 may receive a single pole double throw switch such as indicated at 90 in FIGURE 4, and a hole 91 in panel 10 may receive a potentiometer such as indicated at 92 in FIGURE 4.

Adjustment 0 the Filter If the filter is to operate in the 1000 cycle region, for example, the filter may be adjusted as follows. With the rotatable magnet 70 or 70 removed, the length of reed 25 is adjusted until it resonates at approximately 925 cycles per second. Reed 26 is then adjusted to resonate at 1000 cycles per second by suitably adjusting its effective length, again through the use of screws 28 and 29.

Next, adjust screws 40, 50 and 51, FIGURE 2, to move coils 57, 58 and 66 and their cores 55, 56 and 65 until the space between the reeds 25 and 26 and the nonmagnetic coatings 60, 61 and 67 is about .005 inch. Coils 57 and 58 are as nearly identical as it is possible to make them.

Referring to FIGURE 4, the total effective resistance provided by potentiometer 92 and fixed resistor 94 is equal to the resistance of fixed resistor 95.

With the adjustments properly made, the output at 750 cycles per second and at 1250 cycles. per second will be 40 decibels taking the output at 100 cycles per second as a refernece (zero decibels).

The permanent magnet 70 or 70" is now installed and positioned as indicated for the magnet 70 in FIGURES 1 and 2 so that the field of the permanent magnet impinges on the filter magnetic circuits in the region of the reeds 25 and 26. When the magnet is rotated so that its field reinforces the magnetic flux in the filter magnetic circuits in the region of the reeds 25 and 26, the reeds will vibrate over a wider band of frequencies and the output of the filter will have a wider band width. For example, the output frequency components may cover a range of 300 cycles per second. On the other hand, if the magnet 70 or 70 is rotated by knob 80 until its field cancels part of the magnetic flux in the filter magnetic circuits in the region of the reeds, the band width will be sharper and may effectively be limited to a range of 90 cycles per second.

Referring to FIGURE 4, coils 57 and 58 are connected so that undesired frequency components cancel. In other words, if the reeds 25 and 26 are held against movement, and an input signal supplied to the coil 66, the net output at the jack 88 will be zero if potentiometer 92 is set to supply a maximum value of resistance in series with resistor 94 so that the sum of resistance introduced by potentiometer 92 and resistor 94 is equal to the resistance of resistor 95.

When it is desired to inject all of the input frequencies into the output circuit, potentiometer 92 is adjusted to introduce a reduced value of resistance such that the desired amount of injection is produced. Capacitor 98 tunes the circuit to approximately 950 cycles per second in order to improve the filtering action. When the filter is not to be used, switch 90, FIGURE 4, may be switched to its upper position to connect an input element 99 such 'as the secondary of a transformer directly to the originm load 100 which may comprise the primary of a transformer, for example.

When the switch 90 is in its lower position, the jack 8 8 may be utilized to connect the output of the filter to headphones or to the input of a suitable amplifier or the like.

The non-magnetic coatings 60, 61 and 67 on the cores keep the reeds 25 and 26 from sticking to the cores 55, 56 and 65. Also, the coatings will limit the output. When headphones are used with the filter, it is desirable to limit the output amplitude in order to prevent the sound from causing discomfort to the ears.

The reeds 25 md 26 may be made from a semi-soft steel such as a material used for audio transformer laminations.

The movable permanent magnet 70 or 70 may be mounted in a number of difierent ways, for example a suitable mounting may permit sliding or rolling of a permanent magnet into and out of the operative position such as generally indicated in FIGURE 1. For reasons of economy, resistors 94 and 95 and potentiometer 92 may be omitted. When this'is done, the input siganl may be injected into the output circuit by moving coil 66 to the right or to the left a small amount. Such an adjustment may be accommodated by providing enlarged openings '4 through the mounting plate 32 for the mounting screws 38 and 39 so that by loosening the screws 38 and 39 the adjustment plate 37 may be shifted in a horizontal plane together with the core 65 and coil 66 to the left and right as viewed in FIGURE 4, or as viewed in FIG- URE 2.

Also, for reasons of economy, the movable magnet 70 or 70 may be omitted, but in this case the band width Will be fixed as determined by the reed adjustments and other parameters. Capacitor 98 may also be omitted under some circumstances.

The permanent magnet v23; may be replaced by a block of low retentivity magnetic material such as soft steel.

Summary of Operation With the filter properly adjusted as just described, an input signal is supplied to the input element 99 having a relatively wide range of audio frequency components. If, for example, the filter is to pass frequencies in the region of 1000 cycles per second and deliver the same to an output system associated with jack 38, reed 25 will have an effective length to resonate at 925 cycles per sec- 0nd and reed 26 will have an effective length to resonate at 1000 cycles per second. With the present filter construction adjusted as above described, the output at jack 88 for frequency components of 750 cycles per second and below and at 1250 cycles per second and above will be at least 40 decibels below the output level at 1000 cycles per second.

By rotation of the permanent magnet 70, the range of frequencies which are effectively delivered to the output jack 88 may be adjusted. For example, with the permanent magnet producing a magnetic field which reinforces the flux in the region of the reeds 25 and 26, the normal band pass range may be widened to 300 cycles per second, while if the permanent magnet 70 provides a magnetic field cancelling part of the fiux linking the output windings 57 and 58, the effective band width will be sharpened to a range such as 90 cycles per second (centering about 1000 cycles per second).

When it is desired to inject the input frequencies from source 99 into the output circuit including jack 88, potentiometer 92 is adjusted to reduce the resistance of components 92 and 94 as compared to the resistance of resistor 95.

The non-magnetic coatings 60, 61 and 67 prevent sticking of the reeds to the coil cores and limit the head output so that headphones connected to the output jack 88 will not excessive an excess amplitude which would cause discomfort to the ears.

If the filtering action is not desired, switch 90, FIGURE 4 is switched to the upper position, to complete the normal coupling between elements 99 and 100 of a radiotelegraph receiver or the like.

The ability to change band width readily as by means of the knob 80, FIGURE 2, to inject the input frequencies into the output circuit by adjustment of potentiometer 92, FIGURE 4, and to limit the output amplitude by means of the non-magnetic coatings 60, 61 and 67 represent very desirable features when the filter is used with the radio-telegraph receiver, for example.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of this invention. By way of example, coil 66 may be used as the output coil and coils 57 and 58 maybe used as the input coils.

I claim as my invention:

1. An audio frequency band pass filter comprising (a) a base of magnetic material,

(b) a permanent magnet secured to said base and having one pole adjacent said base and an opposite pole remote from said base,

(0) a pair of reeds of magnetic material having fixed ends adjacent said opposite pole of said permanent magnet and having free ends extending in spaced relation to said base,

(d) said reeds being resonant at respective audio frequencies which together define a desired band pass Width,

(6) output coil means having core means extending from said base toward the free ends of the respective reeds and forming parts of respective magnetic circuits with said reeds and said permanent magnet,

(f) reed driving means coupled to said reeds for driving said reeds in accordance with an input signal to be filtered and comprising input coil means having core means terminating in close relation to the free ends of said reeds, and

(g) one of said coil means comprising a single coil having a core of oblong cross section disposed transversely to said reeds and in magnetic coupling relation to the free ends of said reeds.

2. An audio frequency band pass filter comprising a plurality of reed means having vibrating portions of magnetic material and being resonant at respective audio frequencies which together define a desired band pass width, output coil means coupled to the vibrating portions of the respective reed means, means for producing a unidirection magnetic flux threading said output coil means under the control of the vibrating portions of said reed means, an input circuit comprising input coil means coupled to said reed means for driving said reed means in accordance with an input signal to be filtered, said input coil means driving said reed means in accordance with desired input signal frequencies of said input signal within said band pass width to produce a modulation of the unidirectional magnetic flux threading said output coil means, said input coil means being responsive to undesired input signal frequencies of said input signal outside said desired band pass width to produce magnetic flux components of corresponding undesired frequencies threading said output coil means and producing spurious voltages therein, output terminals connected with said output coil means for providing an output signal from said filter, at least one of said input and output coil means comprising an even number of coils coupled to respective ones of said reed means, and means connecting said even number of coils in balanced opposing relation with respect to undesired input signal frequencies to balance out any spurious voltages at said output terminals, the number of coils of said one of said input and output coil means being not greater than the number of vibrating portions of said reed means, and the output coil means being conductively isolated from said input coil means to prevent current flow from said input circuit through said output coil means, and adjustable permanent magnet means coupled to said vibrating portions of magnetic material for supplying to said vibrating portions an adjustable magnetic flux separate from and additional to said unidirectional magnetic fiuX from said producing means, said adjustable permanent magnet means having manually operable means for selectively incrementally adjusting the position of said permanent magnet means to selectively and adjustably modify the band pass width of said filter.

3. The filter of claim 2 wherein said adjustable permanent magnet means comprises a permanent magnet mounted for rotation adjacent said reed means, said manually operable means being operative to rotate said permanent magnet to adjust the spacing of the poles of said permanent magnet from said reed means.

4. An audio frequency band pass filter comprising a plurality of reed means having vibrating portions of magnetic material and being resonant at respective audio frequencies which together define a desired band pass width, output coil means coupled to the vibrating portions of the respective reed means, means for producing a unidirectional magnetic flux threading said output coil means under the control of the vibrating portions of said reed means, an input circuit comprising input coil means coupled to said reed means for driving said reed means in accordance with an input signal to be filtered, said input coil means driving said reed means in accordance with desired input signal frequencies of said input signal within said band pass width to produce a modulation of the unidirectional magnetic flux threading said output coil means, said input coil means being responsive to undesired input signal frequencies of said input signal outside said desired band pass width to produce magnetic flux components of corresponding undesired frequencies threading said output coil means and producing spurious voltages therein, output terminals connected with said output coil means for providing an output signal from said filter, at least one of said input and output coil means comprising an even number of coils coupled to respective ones of said reed means, means connecting said even number of coils in balanced opposing relation with respect to undesired input signal frequencies to balance out any spurious voltages at said output terminals, the number of coils of said one of said input and output coil means being not greater than the number of vibrating portions of said reed means, and the output coil means being conductively iso lated from said input coil means to prevent current flow from said input circuit through said output coil means, and means for mechanicallylimiting the amplitude of vibration of said vibrating portions of said reed means comprising non-magnetic material engageable with said reed means and spaced therefrom about .005 inch.

5. An audio frequency band pass filter comprising a plurality of reed means having vibrating portions of magnetic material and being resonant at respective audio frequencies which together define a desired band pass Width, output coil means coupled to the vibrating portions of the respective reed means, means for producing a unidirectional magnetic flux threading said output coil means under the control of the vibrating portions of said reed means, an input circuit comprising input coil means coupled to said reed means for driving said reed means in accordance with an input signal to be filtered, said input coil means driving said reed means in accordance with desired input signal frequencies of said input signal Within said band pass width to produce a modulation of the unidirectional magnetic fiux threading said output coil means, said input coil means being responsive to undesired input signal frequencies of said input signal outside said desired band pass width to produce magnetic flux components of corresponding undesired frequencies threading said output coil means and producing spurious voltages therein, output terminals connected with said output coil means for providing an output signal from said filter, at least one of said input and output coil means comprising an even number of coils coupled to respective ones of said reed means, and means connecting said even number of coils in balanced opposing relation with respect to undesired input signal frequencies to balance out any spurious voltages at said output terminals, the number of coils of said one of said input and output coil means being not greater than the number of vibrating portions of said reed means, and the output coil means being conductively isolated from said input coil means to prevent current flow from said input circuit through said output coil means, said output coil means having core means of magnetic material terminating in spaced relation to said vibrating portions of said reed means, and said core means having non-magnetic coating material thereon spaced from said vibrating portions approximately .005 inch and engageable with said vibrating portions to mechanically limit the amplitude of vibration of said reed means.

6. An audio frequency band pass filter comprising a plurality of reed means having vibrating portions of mag netic material and being resonant at respective audio frequencies which together define a desired band pass width, output coil means coupled to the vibrating portions of the respective reed means, means for producing a unidirectional magnetic flux threading said output coil means under the control of the vibrating portions of said reed means, an input circuit comprising input coil means coupled to said reed means for driving said reed means in accordance with an input signal to be filtered, said input coil means driving said reed means in accordance with desired input signal frequencies of said input signal Within said band pass width to produce a modulation of the unidirectional magnetic flux threading'said output coil means, said input coil means being responsive to undesired input signal frequencies of said input signal outside said desired band pass width to produce magnetic flux components of corresponding undesired frequencies threading said output coil means and producing spurious voltages therein, output terminals connected with said output coil means for providing an output signal from said filter, at least one of said input and output coil means comprising an even number of coils coupled to respective ones of said reed means, and means connecting said even number of coils in balanced opposing relation with respect to undesired input signal frequencies to balance out any spurious voltages at said output terminals, the number of coils of said one of said input and output coil means being not greater than the number of vibrating portions of said reed means, and the output coil means being conductively isolated from said input coil means to prevent current flow from said input circuit through said output coil means, and one of said input and output coil means comprising a single coil having a core of oblong cross section disposed transversely to said reed means and in magnetic coupling relation to the vibrating portions of the reed means.

7. An audio frequency band pass filter comprising a plurality of reed means having vibrating portions of magnetic material and being resonant at respective audio frequencies Which together define a desired band pass width, coil means coupled to the vibrating portions of the respective reed means, means for producing a unidirectional magnetic flux threading said coil means under the control of the vibrating portions of said reed means, an input circuit coupled to said reed means for driving said reed means in accordance with an input signal to be filtered, said input circuit driving said reed means in accordance with desired input signal frequencies of said input signal Within said band pass Width to produce a modulation of the unidirectional magnetic flux threading said coil means, said input circuit being responsive to undesired input signal frequencies of said input signal outside said desired band pass width to produce magnetic flux components of corresponding undesired frequencies threading said coil means, output terminals connected with said coil means for providing an output signal from said filter, said coil means comprising an even number of-coils coupled to respective ones of said reed means, and means connecting said even number'of coils in balanced opposingrelation with respect to undesired input signal frequencies to balance out said undesired input signal frequencies at said output terminals, and-adjustable permanent magnet means coupled -to said vibrating portions of magnetic material for supplying to said vibrating portions an adjustable magnetic flux separate from and additional to said unidirectional magnetic flux from said producing means, said adjustable permanent magnet means having manually operable means for selectively incrementally adjusting the position of said permanent magnet means to selectively and adjustable modify the band pass width of said filter.

References Cited in the file of this patent UNITED STATES PATENTS 1,821,181 Gumm Sept. 1, 1931 1,887,713 Gumm Nov. 15, 1932 2,143,437 Fisher Jan. 10, 1939 2,148,578 Pullis Feb. 28, 1939 2,160,876 Lakatos June 6, 1939 2,164,541 Och July 4, 1939 2,222,796 Devol Nov. 26, 1940 2,928,057 Targer Mar. 8, 1960 2,981,905 Mason Apr. 25, 196 1 FOREIGN PATENTS 356,803 Great Britain Sept. 14, 1931 

7. AN AUDIO FREQUENCY BAND PASS FILTER COMPRISING A PLURALITY OF REED HAVING VIBRATING PORTIONS OF MAGNETIC MATERIAL AND BEING RESONANT AT RESPECTIVE AUDIO FREQUENCIES WHICH TOGETHER DEFINED A DESIRED BAND PASS WIDTH, COIL MEANS COUPLED TO THE VIBRATING PORTIONS OF THE RESPECTIVE REED MEANS, MEANS FOR PRODUCING A UNIDIRECTIONAL MAGNETIC FLUX THREADING SAID COIL MEANS UNDER THE CONTROL OF THE VIBRATING PORTIONS OF SAID REED MEANS, AN INPUT CIRCUIT COUPLED TO SAID REED MEANS FOR DRIVING SAID REED MEANS IN ACCORDANCE WITH AN INPUT SIGNAL TO BE FILTERED, SAID INPUT CIRCUIT DRIVING SAID REED MEANS IN ACCORDANCE WITH DESIRED INPUT SIGNAL FREQUENCIES OF SAID INPUT SIGNAL WITHIN SAID BAND PASS WIDTH TO PRODUCE A MODULATION OF THE UNIDIRECTIONAL MAGNETIC FLUX THREADING SAID COIL MEANS, SAID INPUT CIRCUIT BEING RESPONSIVE TO UNDERSIRED INPUT SIGNAL FREQUENCIES OF SAID INPUT SIGNAL OUTSIDE SAID DESIRED BAND PASS WIDTH TO PRODUCE MAGNETIC FLUX COMPONENTS OF CORRESPONDING UNDESIRED FREQUENCIES THREADING SAID 